This invention relates in general to switch mode power supplies and specifically to automatic voltage doubler circuits for such power supplies.
U.S. Pat. No. 4,665,323 entitled ELECTRONICALLY SWITCHABLE POWER SOURCE issued May 12, 1987 in the names of Randy Russell, Wayne Salata and Dennis Stevens and assigned to Zenith Electronics Corporation, describes a DC power source that automatically adapts to either a 120 volt AC or 240 volt AC line input to produce a single range of regulated output voltage. The present circuit accomplishes substantially the same result as the patented circuit with significantly fewer parts, requires much less space and has much lower power consumption. The result is a very attractive automatic voltage doubler circuit arrangement.
The patented circuit utilizes a plurality of comparators for determining when the circuit should switch to the voltage doubler mode. It includes a Triac that is supplied with a DC trigger voltage for switching between the two operating modes. In contrast, the present invention uses a Triac that is supplied with short duration, high energy trigger pulses from a relaxation oscillator for operating the power supply in its voltage doubler mode. The relaxation oscillator is inhibited when the power supply operates in the direct mode.